This invention relates generally to a method and apparatus for removing a liquid from a two-phase flow, and more particularly to the separation of water from a stream of high pressure steam flowing through an exhaust pipe of a high pressure steam turbine.
When a liquid is entrained in a gaseous stream, droplets of that liquid can cause severe erosion within the pipes which carry the gas at high velocities. In a steam turbine system, the problem of pipe erosion is most commonly seen within the pipes which connect the high pressure turbine exhaust to a moisture separator or moisture separator reheater (hereinafter referred to collectively as "moisture separator"). There may be applications where because of space limitations, on warships for example, the high pressure turbine exhaust flow passes directly to the low pressure turbine through crossunder or crossover pipes. Within these pipes, erosion is often most pronounced downstream of where the extraction pipes are joined to crossunder pipes. Such extraction pipes are generally located a short distance below the high pressure turbine exhaust snouts. The erosion of crossunder piping is, therefore, a serious concern to electrical utilities. When pipe erosion causes minor damage, it requires periodic weld repair, mostly in the form of cladding with an erosion resistant material, but, in more severe cases, patches have to be added to the outer surface of the eroded pipes. In some cases replacement of sections or all of the crossunder pipes may be required.
Pipe erosion can be significantly reduced by reducing the amount of entrained moisture in the stream of high pressure steams flowing out of the exhaust snout of the turbine. If entrained moisture is removed from the exhaust steam of the turbine, two important advantages can be realized. The erosion damage to downstream piping can be significantly reduced, and the efficiency of the moisture separator section of the moisture separator reheater may be improved, depending upon its operating effectiveness. In the case of applications where, because of space constraints, there is no moisture separator in the piping between the high pressure and low pressure turbine, application of a moisture separating means according to the teachings of this invention would reduce low pressure turbine inlet moisture and thereby improve turbine efficiency as well as reduce pipe erosion.
One prior art approach for separating liquid from vapor or gas is disclosed in U.S. Pat. No. 4,283,206, issued Aug. 11, 1981 to Andro et al. Dependent upon a despinning action, the apparatus disclosed in Andro et al is comprised generally of an outer vertical tube for admission of a mixture of the vapor or gas and a liquid for separation which mixture is caused to spin and flow downwards. The component further comprises a coaxial inner tube for collecting dry vapor or gas, provided with means for despinning the flow of the dry vapor or gas, the lower edge of the outer tube being at a lower level than that of the upper edge of the inner tube, with orifices being formed in the periphery of the lower edge of the outer tube whose width decreases upwards, and parts for despinning the liquid which drops by gravity along the wall of the outer tube. One problem with the above-described apparatus, however, occurs when water surges and locally high concentrations of moisture form. In such circumstances, the moisture can no longer be separated out to a significant degree. In addition, there are pressure losses resulting from the spinning-despinning.
It is also known from European Patent Application No. 0 096 916 A1, assigned to Brown, Boveri & Cie., to provide in a high-speed water separator, upstream of the deflection blades, a water preseparator which essentially consists of a continuous slit in the wall of the pipe elbow, which slit is overlapped by a cover plate which projects into the flow channel. Although this achieves a separation of the water flowing in the vicinity of the pipe wall, "peeling" of the wall wetness concentration can be only very small if, as intended, only water in laminar flow is to be dealt with.
As is known as well from U.S. Pat. No. 4,527,396, issued on July 9, 1985 to George J. Silvestri, Jr., assigned to the assignee of the present invention and incorporated herein by reference, a moisture separator which incorporates an inner cylinder disposed in coaxial relation with an exhaust pipe of a steam turbine can utilizes the spiral secondary flow of a gas stream to remove liquids which are entrained therein. The moisture separator incorporates an inner cylinder which has one or more apertures through its wall, with the inner cylinder being placed in coaxial relation with the exhaust pipe of a steam turbine with means for sealing the axial ends of an annular chamber formed between the cylinder and the exhaust pipe. Means are provided for dividing the annular chamber into a plurality of arcuate spaces and for removing liquid which collects within each of the arcuate spaces. The moisture separator, thus, utilizes the phenomenon which creates spiral secondary flows when forced to turn around a bend. The spiral flows cause liquid which is entrained in a gas stream, to migrate to the inner surface of a pipe or cylinder and coalesce on the walls thereof. As a result, this moisture separator design utilizes these characteristics of turning streams of gas in order to separate liquid from a moisture-laden gas stream.
Such apparatus, however, is not as efficient as is necessary to remove moisture in the vicinity of a T junction. As is known from P. J. Azzopardi and P. J. Whalley, "The Effect of Flow Patterns on Two-Phase Flow in a T Junction," International Journal of Multiphase Flow, Vol, 8, No. 5, pp. 491-507 (1982), a relatively large proportion of water entrained in a two-phase flow of exhaust steam in piping from a turbine can be removed by a side tube, such as an extraction pipe in conventional crossunder piping within a nuclear power plant, when a modest amount of gas flow is extracted. High speed films of such junctions show an area of high entrainment of the liquid into the gas flow just downstream of the side arm. The entrainment from this area occurs in bursts which have a higher frequency than the natural disturbance waves in the main flow. This extra entrainment occurs because of the thickening of the film caused by the gathering of liquid at that point and the locally lower gas velocity. The liquid gathers at that point because the gas entering the side arm drags part of the film around, and not all of this liquid is successful in entering the side arm. The extra entrainment has important implications for the flow in the main tube and any subsequent takeoff point.
In conventional nuclear power plants having dump systems connected to their crossunder piping, however, it has also been noted that the moisture separator reheater immediately preceded by the dump line would have a higher inlet face erosion than one not having the dump line forming a T junction. Such erosion would, thus, be caused by the collection and entrainment of water as observed by Azzopardi et al on the downstream side of the opening for the side tube when the dump system was activated, but also by the side tube's being filled up with water when the dump system was inactive thereby permitting water droplets to be stripped off from the filled side tube by the high velocity steam.
In order to overcome such disadvantages, a still further prior art approach for a preseparator in a steam turbine installation was disclosed in U.S. Pat. No. 4,624,111, issued Nov. 25, 1986 to Helmut V. Lang, which is assigned to BBC Brown, Boveri and Co., Ltd., Baden, Switzerland. The Lang preseparator includes a first internal pipe positioned within an outer pipe so as to form an interspace therebetween and a second internal pipe positioned between the other two pipes so as to divide the interspace int chambers. The first internal pipe forms a constricted passage through the preseparator and its upstream end is spaced from the outer pipe so as to form an annular gap of isokinetic size. As is taught by the Lang patent, a major proportion of the entrained water flows in the vicinity of the wall of the delivery pipe. This preexisting phase separation is exploited at the annular gap, whose dimensioning separates water-laden steam along the walls from the remainder of the working steam. The pipes further cooperate to effect separation of the water from the water-laden steam, with water being evacuated from one of the two interspaced chambers through a first port and steam being evacuated through a second port. The Lang preseparator, however, neglects to utilize the liquid concentrating mechanism of the side tube or extraction pipe and the water build up immediately downstream of the extraction pipe opening. Furthermore, the Lang preseparator necessitates costly and extensive piping modifications in order to be installed within existing systems. For example, a conventional crossunder piping system having two 18-inch diameter extraction pipes must typically be replaced with four 20-inch diameter pipes that are routed to a large tank where the water and steam are separated, the steam going to a feedwater heater and the water being routed to a moisture separator reheater drain tank or discharge line. Because of such extensive piping modifications and tankage, installation of the Lang preseparator often requires two nuclear plant refueling outages to be completed. Moreover, as a result of not only the reduction of the internal flow area in the Lang preseparator, but also the less than ideal contours of its insert, a conventional nuclear power plant which utilizes such preseparators will incur a loss of power output due to an increase in the pressure drop experienced within the crossunder piping.